Static Dependency injection for Swift.

static-di.swift

import Foundation

protocol Logger {
	func write(_ message: String)
}

final class SystemLogger: Logger {
	func write(_ message: String) {
		print("System: " + message)
	}
}

final class TextLogger: Logger {
	func write(_ message: String) {
		print("Text: " + message)
	}
}

protocol UsesLogger {
	var logger: Logger { get }
}

protocol MixinSystemLogger {}
extension MixinSystemLogger {
	var logger: Logger { return SystemLogger() }
}

protocol MixinTextLogger {}
extension MixinTextLogger {
	var logger: Logger { return TextLogger() }
}

final class Client: UsesLogger, MixinSystemLogger {
	func test() {
		logger.write("test")
	}
}

// For test.

// struct MockLogger: Logger {
// 	func write(_ message: String) {
// 		print("Mock: " + message)
// 	}
// }

// protocol Mock {}

// extension MixinSystemLogger where Self: Mock {
// 	var logger: Logger { return MockLogger() }
// }

// extension Client: Mock {}

// Run!

let client = Client()
client.test()